- Florence, Ronald.
The Perfect Machine.
New York: Harper Perennial, 1994.
ISBN 978-0-06-092670-0.
-
George Ellery Hale
was the son of a wealthy architect and engineer who made his fortune
installing passenger elevators in the skyscrapers which began
to define the skyline of Chicago as it rebuilt from the great
fire of 1871. From early in his life, the young Hale was fascinated
by astronomy, building his own telescope at age 14. Later he would
study astronomy at MIT, the Harvard College Observatory, and in
Berlin. Solar astronomy was his first interest, and he
invented new instruments for observing the Sun and discovered
the magnetic fields associated with sunspots.
His work led him into an academic career, culminating in his
appointment as a full professor at the University of Chicago in
1897. He was co-founder and first editor of the
Astrophysical Journal, published continuously since
1895. Hale's greatest goal was to move astronomy from
its largely dry concentration on
cataloguing stars and measuring planetary positions
into the new science of astrophysics: using observational
techniques such as
spectroscopy
to study the composition of stars and nebulæ and, by
comparing them, begin to deduce their origin, evolution,
and the mechanisms that made them shine. His own work on solar
astronomy pointed the way to this, but the Sun was just one star.
Imagine how much more could be learned when the Sun was compared
in detail to the myriad stars visible through a telescope.
But observing the spectra of stars was a light-hungry process,
especially with the insensitive photographic material available
around the turn of the 20th century. Obtaining the spectrum of all
but a few of the brightest stars would require exposure times so
long they would exceed the endurance of observers to operate the
small telescopes which then predominated, over multiple nights.
Thus, Hale became interested in larger telescopes, and the quest
for ever more light from the distant universe would occupy him for
the rest of his life.
First, he promoted the construction of a 40 inch (102 cm)
refractor telescope, accessible from Chicago at a dark sky site
in Wisconsin. At the epoch, universities, government, and
private foundations did not fund such instruments. Hale
persuaded Chicago streetcar baron Charles T. Yerkes to pick
up the tab, and
Yerkes
Observatory was born. Its 40 inch refractor remains the
largest telescope of that kind used for astronomy to this day.
There are two principal types of astronomical telescopes. A
refracting telescope
has a convex lens at one end of a tube, which focuses
incoming light to an eyepiece or photographic plate at
the other end. A
reflecting telescope
has a concave mirror at the bottom of the tube, the top end of
which is open. Light enters the tube and falls upon the mirror, which
reflects and focuses it upward, where it can be picked off by
another mirror, directly focused on a sensor, or bounced back down
through a hole in the main mirror. There are a multitude of
variations in the design of both types of telescopes, but the
fundamental principles of refraction and reflection remain the same.
Refractors have the advantages of simplicity, a sealed tube
assembly which keeps out dust and moisture and excludes air currents
which might distort the image but, because light passes through the
lens, must use clear glass free of bubbles, strain lines, or other
irregularities that might interfere with forming a perfect
focus. Further, refractors tend to focus different colours of
light at different distances. This makes them less
suitable for use in spectroscopy. Colour performance can be improved
by making lenses of two or more different kinds of glass (an
achromatic
or
apochromatic
design), but this further increases the complexity, difficulty, and
cost of manufacturing the lens. At the time of the construction of
the Yerkes refractor, it was believed the limit had been reached for
the refractor design and, indeed, no larger astronomical refractor
has been built since.
In a reflector, the mirror (usually made of glass or some glass-like
substance) serves only to support an extremely thin (on the order of
a thousand atoms) layer of reflective material (originally silver, but
now usually aluminium). The light never passes through the glass at
all, so as long as it is sufficiently uniform to take on and hold the
desired shape, and free of imperfections (such as cracks or bubbles)
that would make the reflecting surface rough, the optical qualities
of the glass don't matter at all. Best of all, a mirror reflects all
colours of light in precisely the same way, so it is ideal for
spectrometry (and, later, colour photography).
With the Yerkes refractor in operation, it was natural that
Hale would turn to a reflector in his quest for ever more light.
He persuaded his father to put up the money to order a 60 inch
(1.5 metre) glass disc from France, and, when it arrived months
later, set one of his co-workers at Yerkes, George W. Ritchey,
to begin grinding the disc into a mirror. All of this was on
speculation: there were no funds to build a telescope,
an observatory to house it, nor to acquire a site for
the observatory. The persistent and persuasive Hale approached
the recently-founded Carnegie Institution, and eventually secured
grants to build the telescope and observatory on Mount Wilson
in California, along with an optical laboratory in nearby Pasadena.
Components for the telescope had to be carried up the crude trail
to the top of the mountain on the backs of mules, donkeys, or
men until a new road allowing the use of tractors was built.
In 1908 the sixty inch telescope began operation, and its optics
and mechanics performed superbly. Astronomers could see much
deeper into the heavens. But still, Hale was not satisfied.
Even before the sixty inch entered service, he approached
John D. Hooker, a Los Angeles hardware merchant, for seed money
to fund the casting of a mirror blank for an 84 inch telescope,
requesting US$ 25,000 (around US$ 600,000 today). Discussing
the project, Hooker and Hale agreed not to settle for 84, but
rather to go for 100 inches (2.5 metres). Hooker pledged
US$ 45,000 to the project, with Hale promising the telescope
would be the largest in the world and bear Hooker's name. Once
again, an order for the disc was placed with the Saint-Gobain
glassworks in France, the only one with experience in such large
glass castings. Problems began almost immediately. Saint-Gobain
did not have the capacity to melt the quantity of glass required (four
and a half tons) all at once: they would have to fill the mould
in three successive pours. A massive piece of cast glass (101
inches in diameter and 13 inches thick) cannot simply be allowed
to cool naturally after being poured. If that were to occur,
shrinkage of the outer parts of the disc as it cooled while the
inside still remained hot would almost certainly cause the disc to
fracture and, even if it didn't, would create strains within the disc
that would render it incapable of holding the precise figure
(curvature) required by the mirror. Instead, the disc must
be placed in an
annealing
oven, where the temperature is reduced slowly over a period of time,
allowing the internal stresses to be released. So massive was
the 100 inch disc that it took a full year to anneal.
When the disc finally arrived in Pasadena, Hale and Ritchey were
dismayed by what they saw, There were sheets of bubbles between
the three layers of poured glass, indicating they had not
fused. There was evidence the process of annealing had caused the
internal structure of the glass to begin to break down. It seemed
unlikely a suitable mirror could be made from the disc. After
extended negotiations, Saint-Gobain decided to try again, casting a
replacement disc at no additional cost. Months later, they reported
the second disc had broken during annealing, and it was likely no
better disc could be produced. Hale decided to proceed with the
original disc. Patiently, he made the case to the Carnegie
Institution to fund the telescope and observatory on Mount Wilson. It
would not be until November 1917, eleven years after the order was
placed for the first disc, that the mirror was completed, installed in
the massive new telescope, and ready for astronomers to gaze through
the eyepiece for the first time. The telescope was aimed at brilliant
Jupiter.
Observers were horrified. Rather than a sharp image,
Jupiter was smeared out over multiple overlapping images, as if
multiple mirrors had been poorly aimed into the eyepiece. Although
the mirror had tested to specification in the optical shop, when
placed in the telescope and aimed at the sky, it appeared to be
useless for astronomical work. Recalling that the temperature
had fallen rapidly from day to night, the observers adjourned until
three in the morning in the hope that as the mirror continued to
cool down to the nighttime temperature, it would perform better.
Indeed, in the early morning hours, the images were superb. The
mirror, made of ordinary plate glass, was subject to thermal expansion
as its temperature changed. It was later determined that the massive
disc took twenty-four hours to cool ten degrees Celsius.
Rapid changes in temperature on the mountain could
cause the mirror to misbehave until its temperature stabilised.
Observers would have to cope with its temperamental nature throughout
the decades it served astronomical research.
As the 1920s progressed, driven in large part by work done on
the 100 inch Hooker telescope on Mount Wilson, astronomical
research became increasingly focused on the “nebulæ”,
many of which the great telescope had revealed were
“island universes”, equal in size to our own Milky
Way and immensely distant. Many were so far away and faint that
they appeared as only the barest smudges of light even in long
exposures through the 100 inch. Clearly, a larger telescope was
in order. As always, Hale was interested in the challenge. As
early as 1921, he had requested a preliminary design for a three
hundred inch (7.6 metre) instrument. Even based on early sketches,
it was clear the magnitude of the project would surpass any
scientific instrument previously contemplated: estimates came
to around US$ 12 million (US$ 165 million today). This was before
the era of “big science”. In the mid 1920s, when Hale
produced this estimate, one of the most
prestigious scientific institutions in the world,
the Cavendish Laboratory
at Cambridge, had an annual research budget of less than
£ 1000 (around US$ 66,500 today). Sums in the millions and
academic science simply didn't fit into the same mind, unless
it happened to be that of George Ellery Hale. Using his connections,
he approached people involved with foundations endowed by the
Rockefeller fortune. Rockefeller and Carnegie were competitors
in philanthropy: perhaps a Rockefeller institution might be interested
in outdoing the renown Carnegie had obtained by funding the largest
telescope in the world. Slowly, and with an informality which seems
unimaginable today, Hale negotiated with the Rockefeller foundation,
with the brash new university in Pasadena which now called itself
Caltech, and with a prickly Carnegie foundation who saw the new
telescope as trying to poach its painfully-assembled technical and
scientific staff on Mount Wilson. By mid-1928 a deal was in hand: a
Rockefeller grant for US$ 6 million (US$ 85 million today) to design and
build a 200 inch (5 metre) telescope. Caltech was to raise the funds
for an endowment to maintain and operate the instrument once it was
completed. Big science had arrived.
In discussions with the Rockefeller foundation, Hale had agreed
on a 200 inch aperture, deciding the leap to an instrument
three times the size of the largest existing telescope and the
budget that would require was too great. Even so, there were
tremendous technical challenges to be overcome. The 100 inch
demonstrated that plate glass had reached or exceeded
its limits. The problems of distortion due to temperature changes
only increase with the size of a mirror, and while the 100 inch was
difficult to cope with, a 200 inch would be unusable, even if it
could be somehow cast and annealed (with the latter process probably
taking several years). Two promising alternatives were
fused quartz
and
Pyrex borosilicate glass.
Fused quartz has hardly any thermal expansion at all. Pyrex has about
three times greater expansion than quartz, but still far less than
plate glass.
Hale contracted with General Electric Company to produce a
series of mirror blanks from fused quartz. GE's legendary
inventor
Elihu Thomson,
second only in reputation to Thomas Edison, agreed to undertake the
project. Troubles began almost immediately. Every attempt to get
rid of bubbles in quartz, which was still very viscous even at
extreme temperatures, failed. A new process, which involved
spraying the surface of cast discs with silica passed through
an oxy-hydrogen torch was developed. It required machinery
which, in operation, seemed to surpass visions of
hellfire. To build up the coating on a 200 inch disc would require
enough hydrogen to fill two Graf Zeppelins. And still,
not a single suitable smaller disc had been produced from fused
quartz.
In October 1929, just a year after the public announcement of
the 200 inch telescope project, the U.S. stock market crashed
and the economy began to slow into the great depression.
Fortunately, the Rockefeller foundation invested very
conservatively, and lost little in the market chaos, so the
grant for the telescope project remained secure. The
deepening depression and the accompanying deflation was a
benefit to the effort because raw material and manufactured
goods prices fell in terms of the grant's dollars, and industrial
companies which might not have been interested in a one-off
job like the telescope were hungry for any work that would
help them meet their payroll and keep their workforce employed.
In 1931, after three years of failures, expenditures billed at
manufacturing cost by GE which had consumed more than one tenth
the entire budget of the project, and estimates far beyond that
for the final mirror, Hale and the project directors decided to
pull the plug on GE and fused quartz. Turning to the alternative
of Pyrex, Corning glassworks bid between US$ 150,000 and 300,000
for the main disc and five smaller auxiliary discs. Pyrex was
already in production at industrial scale and used to make
household goods and laboratory glassware in the millions,
so Corning foresaw few problems casting the telescope discs.
Scaling things up is never a simple process, however, and Corning
encountered problems with failures in the moulds, glass contamination,
and even a flood during the annealing process before the big disc
was ready for delivery.
Getting it from the factory in New York to the optical shop in
California was an epic event and media circus. Schools let out
so students could go down to the railroad tracks and watch the
“giant eye” on its special train make its way across
the country. On April 10, 1936, the disc arrived at the optical
shop and work began to turn it into a mirror.
With the disc in hand, work on the telescope structure and
observatory could begin in earnest. After an extended period
of investigation, Palomar Mountain had been selected as the
site for the great telescope. A rustic construction camp was
built to begin preliminary work. Meanwhile, Westinghouse
began to fabricate components of the telescope mounting, which
would include the largest bearing ever manufactured.
But everything depended on the mirror. Without it, there would
be no telescope, and things were not going well in the optical shop.
As the disc was ground flat preliminary to being shaped into the
mirror profile, flaws continued to appear on its surface. None of
the earlier smaller discs had contained such defects. Could it be
possible that, eight years into the project, the disc would be found
defective and everything would have to start over? The analysis concluded
that the glass had become contaminated as it was poured, and that the
deeper the mirror was ground down the fewer flaws would be discovered.
There was nothing to do but hope for the best and begin.
Few jobs demand the patience of the optical craftsman. The great
disc was not ready for its first optical test until September 1938.
Then began a process of polishing and figuring, with weekly
tests of the mirror. In August 1941, the mirror was judged to have
the proper focal length and spherical profile. But the mirror
needed to be a
parabola,
not a sphere, so this was just the start of
an even more exacting process of deepening the curve. In January 1942,
the mirror reached the desired parabola to within one wavelength
of light. But it needed to be much better than that. The U.S. was
now at war. The uncompleted mirror was packed away “for the
duration”. The optical shop turned to war work.
In December 1945, work resumed on the mirror. In October 1947, it was
pronounced finished and ready to install in the telescope. Eleven
and a half years had elapsed since the grinding machine started to
work on the disc. Shipping the mirror from Pasadena to the mountain
was another epic journey, this time by highway. Finally, all the pieces
were in place. Now the hard part began.
The glass disc was the correct shape, but it wouldn't be a mirror
until coated with a thin layer of aluminium. This was a process which
had been done many times before with smaller mirrors, but as always
size matters, and a host of problems had to be solved before a
suitable coating was obtained. Now the mirror could be installed in
the telescope and tested further. Problem after problem with the
mounting system, suspension, and telescope drive had to be found
and fixed. Testing a mirror in its telescope against a star is much more
demanding than any optical shop test, and from the start of 1949, an
iterative process of testing, tweaking, and re-testing began. A
problem with astigmatism in the mirror was fixed by attaching four
fisherman's scales from a hardware store to its back (they are still
there). In October 1949, the telescope was declared finished and
ready for use by astronomers. Twenty-one years had elapsed since
the project began. George Ellery Hale died in 1938, less than
ten years into the great work. But it was recognised as his
monument, and at its dedication was named the
“Hale Telescope.”
The inauguration of the Hale Telescope marked the end of the rapid
increase in the aperture of observatory telescopes which had
characterised the first half of the twentieth century, largely
through the efforts of Hale. It would remain the largest telescope
in operation until 1975, when the Soviet six metre
BTA-6 went into
operation. That instrument, however, was essentially an
exercise in Cold War one-upmanship, and never achieved its
scientific objectives. The Hale would not truly be surpassed before
the ten metre
Keck I
telescope began observations in 1993, 44 years after the Hale. The
Hale Telescope remains in active use today, performing observations
impossible when it was inaugurated thanks to electronics undreamt
of in 1949.
This is an epic recounting of a grand project, the dawn of “big
science”, and the construction of instruments which
revolutionised how we see our place in the cosmos. There is far
more detail than I have recounted even in this long essay, and
much insight into how a large, complicated project, undertaken
with little grasp of the technical challenges to be overcome,
can be achieved through patient toil sustained by belief in the
objective.
A PBS documentary,
The Journey to Palomar, is
based upon this book. It is available on
DVD or a
variety of streaming services.
In the Kindle edition, footnotes which appear
in the text are just asterisks, which are almost impossible to select
on touch screen devices without missing and accidentally turning the
page. In addition, the index is just a useless list of terms and page
numbers which have nothing to do with the Kindle document, which lacks
real page numbers. Disastrously, the illustrations which appear in the
print edition are omitted: for a project which
was extensively documented in photographs, drawings, and motion
pictures, this is inexcusable.
October 2016 